The present invention relates generally to power transfer systems operable for controlling the distribution of drive torque between a pair of rotary shafts and, more particularly, to a torque transfer clutch assembly equipped with a hydraulic linear piston clutch actuator.
In view of increased consumer demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) to the non-driven wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. A mechanical mode shift mechanism can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheel to the driven wheels in order to establish a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions.
Alternatively, it is known to use xe2x80x9con-demandxe2x80x9d power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, it is known to incorporate the on-demand feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that the drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque xe2x80x9con-demandxe2x80x9d to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement.
Conventional clutch assemblies typically include a clutch pack operably connected between a drive member and a driven member. A power-operated actuator controls engagement of the clutch pack. Specifically, torque is transferred from the drive member to the driven member by actuating the power-operated actuator. The power-operated actuator displaces an apply plate which acts on the clutch pack and increases the frictional engagement between the interleaved plates.
A variety of power-operated actuators have been used in the art. Exemplary embodiments include those disclosed in U.S. Pat. No. 5,407,024 wherein a ball-ramp arrangement is used to displace the apply plate when a current is provided to an induction motor. Another example disclosed in U.S. Pat. No. 5,332,060, assigned to the assignee of the present application, includes a linear actuator that pivots a lever arm to regulate the frictional forces applied to the clutch pack. Neither of these references incorporate a closed hydraulic system to control actuation of the associated clutch. While the above actuator devices may perform adequately for their intended purpose, a need exists for an improved actuator that is less complex, reduces the number of friction generating components which lead to inefficiencies and larger motor requirements, and an annular arrangement that provides operational simplicity and reduced space requirements.
In view of the above, the present invention is directed to a power transfer system for a four-wheel drive vehicle equipped with a torque transfer clutch assembly having a multi-plate friction clutch pack and a hydraulic linear piston clutch actuator. The hydraulic linear piston clutch actuator includes a ball screw assembly having a threaded lead screw and a ball nut. The threaded lead screw is rotated by an electric motor through a reduction gearset for causing linear translation of the ball nut. A control piston is secured to the ball nut for linear movement in a control chamber which, in turn, is in fluid communication with apply chambers to define a closed hydraulic circuit. Multiple apply chambers are radially located about a transfer plate which is rotatably coupled to a clutch apply plate. An apply piston is retained in each apply chamber and is moveable in response to movement of the control piston for exerting a clutch engagement force on the clutch pack. This clutch actuator arrangement yields numerous operational advantages over the prior art including, but not limited to, improved response characteristics with lower hysteresis, superior torque control improved system efficiency, low cost, and weight savings.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description, attached drawings and claims.